In rocket engines it is critical for proper performance that the amount of fuel entering the engine be carefully controlled. Therefore a static face seal is employed around the turbine shaft to prevent fuel leaking into or around the combustion chamber before ignition. Once the engine is started, the static face seal, due to friction, would soon wear out. Therefore, it is removed from contact with the rotating mating ring when the engine starts and is pushed back into contact with the rotating mating ring when the engine is shut down.
A second seal, a floating ring seal, is employed around the turbine shaft to prevent fuel flow-by while the shaft is turning at high speed.
In the past, the static lift-off face seal and its mechanism for movement and the floating seal ring were separate more complex units.